1. Field of the Invention
This invention relates to a sol id-state image pickup apparatus for producing digitized picture data from image pickup signals produced by a solid-state image sensor such as a CCD image sensor made up of charge-coupled devices (CCDs) and outputting the produced digitized picture data. More particularly, it relates to a solid-state image pickup apparatus having a rate converting function of converting the data clock rates of the generated picture data.
2. Description of the Prior Art
It is known in general that, in a solid-state image pickup apparatus having, as image pickup means, a solid-state image sensor having a discrete pixel structure, such as a CCD image sensor, since the solid-state image sensor itself is a sampling system, aliasing components from the spatial sampling frequency tend to be mixed into the image pickup signal from the solid-state image sensor. The conventional practice for preventing the generation of aliasing components into the baseband component of the image pickup signals is to provide a double refraction type optical low-pass filter in the image pickup optical system to suppress high-frequency components of the baseband component of the image pickup signals to satisfy the Nyquist conditions of the sampling system constituted by the solid-state image sensor.
On the other hand, with a color television camera device for imaging a color picture, a multiple CCD plate type solid-state image pickup apparatus, such as a two CCD plate type solid-state image pickup apparatus, for imaging a three-color picture by a solid-state image sensor having for imaging a green-colored picture and a solid-state image sensor having a color coding filter for red-colored and blue-colored pictures, or a three CCD plate type solid-state image pickup apparatus for imaging a three-color picture by separate solid-state image sensors, has been put to practical use.
Besides, as a technique for improving the resolution in the above-described multiple CCD plate type solid-state image pickup apparatus, there is known a spatial pixel shifting method in which the solid-state image sensors for imaging red-colored pictures and blue-colored pictures are shifted with respect to the solid-state image sensor for imaging the red-colored picture by one-half the spatial pixel sampling period. By adopting the spatial pixel shifting method, a high resolution exceeding the threshold of the number of pixels of the solid-state image sensor may be realized with the multiple CCD plate type solid-state image pickup apparatus with an analog output.
On the other hand, a D-1 standard or a D-2 standard is prescribed as the standard for an industrial digital VTR employed in e.g. a telecasting station. Thus a digital interface for a digital video related equipment conforming to these standards has become necessary to provide for a color television apparatus.
It is noted that with the D-1 standard for 4:2:2 digital component video signals, the sampling frequency is set to 13.5 MHz, corresponding to 858 times the horizontal frequency f.sub.H(NTSC) for the NTSC system and to 864 times the horizontal frequency f.sub.H(PAL) for the PAL system, and is adapted for being locked at a frequency equal to an integer number times the horizontal frequency for either systems. On the other hand, with the D-2 standard for the digital composite video signals, the sampling frequency is set to four times the subcarrier frequency to minimize beat interference between the subcarrier and sampling clocks, with the sampling frequency f.sub.S(NTSC) for the NTSC system and the sampling frequency for the PAL system f.sub.S(PAL) being 14.3 MHz and 17.734 MHz, respectively.
Meanwhile, if it is desired to implement a solid-state image pickup apparatus capable of directly outputting digital picture signals conforming to the above-mentioned D-1 and D-2 standards, such digital picture signals being high in resolution and picture quality and containing only little aliasing distortion components, it is necessary that the sampling rate (number of pixels) of the solid-state image sensor employed in the image pickup unit be set so as to be higher than the sampling rate for the D-1 or D-2 standard, in consideration that the optical low-pass filter as a prefilter for the solid-state image sensor is optically not unobjectionable, that is that only smooth roll-off characteristics may be obtained with the optical low-pass filter such that high modulation transfer function (MTF) characteristics may be obtained only at the costs of increase in the aliasing distortion components.
Besides, if account is taken of the fact that correction of pixel-based defects in the image pickup signals by the solid-state image sensor is performed by a digital technique, and the beat interference has to be prevented from occurring, it is desirable that the sampling rate of the solid-state image sensor be coincident with that of the analog-to-digital converting unit adapted for digitizing the image pickup signals supplied by the solid-state image sensor.
The CCD image sensor now in widespread use is driven at the clock rate of 14.3 MHz=f.sub.SC(NTSC). With a digital camera having its image pickup unit constituted by such CCD image sensor, image pickup signals outputted from the solid-state image sensor are digitized at the above-mentioned clock rate of 14.3 MHz=f.sub.SC(NTSC) by way of performing a digital signal processing operation.
However, the clock rate in the D-1 standard, which is the standard for the above-mentioned 4:2:2 digital component video signals, cannot be matched to the clock rate for the above-mentioned digital camera having its image pickup unit constituted by such CCD image sensor, with the luminance signal Y and the color difference signals C.sub.R /C.sub.B for the D-1 standard being 13.5 MHz and 6.75 MHz, respectively. If a CCD image sensor having the readout rate of 13.5 MHz is to be fabricated newly for meeting the D-1 standard, there is raised a problem in connection with costs and limitation in general adaptability.
On the other hand, with the multiple CCD plate type solid-state image pickup apparatus, constructed in accordance with the spatial pixel shifting method, the analog output cannot be improved in resolution unless a signal processing system operated at a clock rate of 2f.sub.s1, which is double the clock rate f.sub.s1 of the CCD image sensor, is employed. Although it may be contemplated to process signals at f.sub.s1 and 2f.sub.s1 and to turn the signals into analog signals at f.sub.s1 and 2f.sub.s1, with the analog signals being then passed through an analog filter so as to be digitized again at the clock rate prescribed by the D-1 standard. However, in such case, beat interference is produced between the 14.3 MHz system and the 13.5 MHz system to incur deterioration picture quality.